For the sake of convenience, the present invention will be explained while distinguishing between high-temperature solder and ordinary solder. High-temperature solder collectively refers to solder having a melting point higher than that of ordinary solder. In this specification, a high-temperature solder means a lead-free, high-temperature solder having a solidus temperature of 255° C. or higher.
A high-temperature solder can be used in the same manner as an ordinary solder, but high-temperature solder is used in applications in which the resulting solder joints are exposed to high temperatures such as those to which electrodes of power transistor equipment are exposed. High-temperature solders can also be used in other applications. Namely, in soldering of electronic equipment, there are cases in which soldering (first soldering) is applied to an intended member, and then additional soldering (second soldering) is applied to the neighborhood of the previously-soldered member. When the second soldering is carried out, the first-prepared solder joint must not be melted. Therefore, a high-temperature solder having a high melting point is used in the first soldering.
Conventional high-temperature solders are Pb-based solders such as Pb-5Sn (solidus temperature: 300° C., liquidus temperature: 314° C.), Pb-10Sn (solidus temperature: 268° C., liquidus temperature: 301° C.), and Pb-2.5Ag (solidus temperature: 304° C., liquidus temperature: 304° C.). These Pb-based, high-temperature solders can exhibit improved solderability with respect to electrodes made of Cu, Ag, and the like. Recently, however, the use of Pb-based solder is being restricted due to pollution problems caused by Pb. At present, lead-free, high-temperature solder is under development, but an Sn-based, lead-free solder which has both solidus and liquidus temperatures high enough to be suitable for a high-temperature solder has not yet been proposed.
Typical lead-free solders at present, such as Sn—Ag—Cu alloys (Sn-3Ag-0.5Cu), are used to carry out soldering of electronic parts to a circuit board and the like at a temperature of about 240° C. When electronic parts are mounted on a circuit board and the like and the electronic parts have been soldered using a lead-free, high-temperature solder, it is necessary for the high-temperature solder to have a solidus temperature of 255° C. or higher from a practical viewpoint, since the resulting solder joints must not be melted during mounting (packaging) carried out by the second soldering. In the past, a high-temperature solder which is in a half-melted state at a temperature of about 255° C. was proposed. This was because there was no high-temperature solder which could be substituted for conventional Pb-containing high-temperature solders, so there was no choice except to use a solder alloy which is not completely melted at a temperature of 255° C.
Bi-based alloys such as Bi—Ag, Bi—Cu, and Bi—Sb are known as alloys which have a solidus temperature of 255° C. or higher (Patent Documents 1-3). However, these solder alloys do not react with an electrode made of Cu, Ag, etc., to which soldering must be applied, and it is difficult to obtain a satisfactory solder joint. In order to promote reaction of a solder alloy with an electrode made of Cu, Ag, etc., the addition of Sn, In and the like is effective, since Sn and In form an intermetallic compound with the metal of which the electrode is made. However, this method has the disadvantage that mere addition of these elements, i.e., the addition of 0.2 mass % or more of Sn or of a minute amount of In would result in a composition having a solidus temperature of 139° C. or 109.5° C., respectively, which is lower than 255° C.
Patent Document 1: JP 2001-205477 A1
Patent Document 2: JP 2005-72173 A1
Patent Document 3: JP-2001-353590 A1